The invention relates to an X-ray system which includes at least one X-ray source which is provided with a control grid and serves to form X-ray images, at least one X-ray image converter which is provided with means for electronically reading out X-ray images, and an X-ray generator for power supply of the X-ray source. The invention also relates to an X-ray generator suitable for an X-ray system of this kind.
When X-ray images are formed, energy is still stored in the capacitances of the system at the end of an X-ray exposure. These capacitances include the capacitances of the cable or cables via which the X-ray source is connected to a high-voltage generator and also the capacitors of a DC/AC converter which is included in the X-ray generator. Because of the stored energy, at the end of the exposure the high voltage on the X-ray source can decrease at the end of the exposure only to the extent to which the capacitances are discharged, that is, mainly via the X-ray source. The discharging of the capacitances via the X-ray source will take more time as the current through the X-ray source during the exposure is smaller. Therefore, the X-ray source continues to emit radiation after the end of the actual exposure; such radiation may give rise to undesirable overexposures.
This problem is serious notably when thin objects are imaged, for example, in pediatrics, because the small thickness of the object and a specified value of the high voltage (for example, 70 kV) permit the switching of only a very small mAs value (approximately 0.05 mAs). Because of the energy stored in the capacitances, however, X-ray sources (without control grid) only allow the switching of mAs values which are a number of times higher than the desired mAs value. These values allow overexposure of an X-ray image to be avoided only if, contrary to, for example, the IEC regulations, the exposure takes place while using a lower voltage across the X-ray tube. However, when the voltage across the X-ray tube is lower, the radiation load for the patient will be higher.
This dilemma is avoided in an X-ray generator which is known from Japanese patent application 11-204289 and is provided with a high-voltage generator, whereto the X-ray source is connected, and also with means for switching the high voltage of the high-voltage generator on and off and with a grid control circuit for controlling the grid. This X-ray generator serves to generate stable X-ray pulses without overshoot with the aid of the control grid. Additionally, at the end of the exposure the current through the X-ray source is switched off by means of the control grid, so that the energy stored in the system cannot give rise to overexposure.
Problems are encountered, however, when an X-ray generator of this known kind has to feed not only the X-ray source provided with a control grid, but also one or more X-ray sources without a control grid. X-ray sources of that kind are used for economical reasons in the case of high exposure powers for which the described problem is not so serious. The problem consists in that the stored energy, or the high voltage across the X-ray source, can decrease only very slowly because the control grid blocks the current flow through the X-ray source after the end of the exposure. When the X-ray generator is switched over to a different X-ray source (of a different apparatus) in this phase, such switching over takes place at a high voltage for which the conventional high voltage switches are not designed. Moreover, it is undesirable that the new X-ray source emits X-rays already at the beginning of the preparation phase which precedes an X-ray exposure and in which, for example, the rotary anode is accelerated to the correct speed and the filament of the cathode of this X-ray source is heated.
The latter problem is also encountered in X-ray tubes which are provided with two cathodes for two focal spots of different size; in that case the electron current to one focal spot (usually the smaller one) can be blocked by means of a control grid and no grid control is available for the other focal spot. In the case of examination of one and the same object, an automatic change-over from one focal spot to the other and vice versa may occur during a series of exposures, thus giving rise to premature emission of X-rays.
It is an object of the invention to provide an X-ray system or an X-ray generator which on the one hand enable accurate exposure of the X-ray image and in which on the other hand the problems involved in the fast change-over from X-ray exposures with grid control to X-ray exposures without grid control are avoided at least to a high degree.
The object in accordance with the invention is achieved by means of an X-ray system which includes at least one X-ray source which is provided with a control grid and serves to form X-ray images, and also includes at least one X-ray image converter which is provided with means for electronically reading out X-ray images or for transporting the X-ray image converter out of the zone covered by the X-ray source in a time interval which succeeds the X-ray exposure, and also an X-ray generator for power supply of the X-ray source, which X-ray generator includes:
a high-voltage generator whereto the X-ray source can be connected,
means for switching the high voltage of the high-voltage generator on and off at the beginning and the end of an X-ray exposure, and
a grid control circuit for blocking the control grid and the current through the X-ray source during the time interval and for subsequently enabling the current to flow through the X-ray source.
In accordance with the invention, the control grid, or the current through the X-ray source, is blocked in the time interval after the end of the exposure in which the X-ray exposure is read out (in the case of an X-ray image converter which is suitable for electronic reading out) or in which the X-ray image converter is moved out of the beam path (in the case of an X-ray image converter in the form of a film-foil combination or a storage phosphor). Consequently, in this time interval the X-rays are interrupted so that further exposure of the X-ray image converter (or overexposure) no longer takes place. Because of the blocking of the X-ray source, the high voltage across the X-ray source decreases only very slowly during this time interval.
When the current through the X-ray source is enabled again after the time interval, X-rays are produced again, but such X-rays are not of importance to the previous X-ray exposure (already electronically read out or transported out of the beam path together with the X-ray image converter). However, the capacitances of the system can then also be discharged via the X-ray source, so that the voltage across the X-ray source decreases substantially faster than during the interruption of the current by means of the control grid. Consequently, problems are no longer encountered when briefly thereafter switching over takes place from one X-ray source or one focal spot to another X-ray source or focal spot.
An X-ray generator for power supply of at least one X-ray source which is provided with a control grid in order to form X-ray images for an X-ray system as described above including a high-voltage generator whereto the X-ray source is connected and means for switching the high voltage of the high-voltage generator on and off at the beginning and the end of an X-ray exposure. A grid control circuit is provided for blocking the control grid and the current through the X-ray source during a short time interval (T2-T3) and for subsequently enabling the current to flow through the X-ray source.
The following description, claims and accompanying drawings set forth certain illustrative embodiments applying various principles of the present invention. It is to be appreciated that different embodiments applying principles of the invention may take form in various components, steps and arrangements of components and steps. These described embodiments being indicative of but a few of the various ways in which some or all of the principles of the invention may be employed in a method or apparatus. The drawings are only for the purpose of illustrating an embodiment of an apparatus and method applying principles of the present invention and are not to be construed as limiting the present invention.